Are there any limits on the number of rows I can merge?

This article details the limits associated with the free version of Mail Merge add-on and the limits imposed by Google on Gmail and Google Docs usage.

Mail Merge Limits

*Subject to Google-imposed limits (see below).

Google limits: Emails

If you send hundreds of emails at once, Gmail may send you an error; this is Gmail's way of trying to discourage spamming.

You should also be aware of Gmail Sending Limits.

Therefore, the following workarounds are suggested:

Google limits: Letters

When you merge more than 100 pages into a single document, you may receive errors as Google Docs is not designed to handle huge content. So we suggest you merge in batches.